A polyethylene-based resin foam is generally used as a packing material owing to its high elasticity and excellent oil and impact resistance. The polyethylene-based resin foam, however, has weakness that its stiffness and compressive strength are low. On the other hand, a styrene-based resin foam is excellent in stiffness, but has a weakness that it is brittle.
To overcome such weakness, Japanese Examined Patent Publication No. SHO 51(1976)-46138 (Patent Document 1), Japanese Examined Patent Publication No. SHO 52(1977)-10150 (Patent Document 2), Japanese Examined Patent Publication No. SHO 58(1983)-53003 (Patent Document 3) and Japanese Unexamined Patent Publication No. SHO 62(1987)-59642 (Patent Document 4) disclose methods in which polyethylene-based resin is impregnated with a styrene-based monomer and polymerization is performed to obtain styrene-modified polyethylene-based expandable resin particles. The polyethylene-based resins used in these methods are mostly low-density polyethylene, high-density polyethylene, and ethylene/vinyl acetate copolymer. Since the styrene-based monomer cannot be sufficiently dispersed in such polyethylene-based resins, the polyethylene-based resin needs to be crosslinked in order to provide sufficient stiffness and impact resistance.
Japanese Patent No. 2668384 (Patent Document 5) discloses a method for obtaining an expanded molded article of modified polyethylene-based resin excellent in stiffness and impact resistance. In this method, 100 parts by weight of non-crosslinked linear low-density polyethylene-based resin particles, 5 to 300 parts by weight of a vinyl aromatic monomer, and 1 to 3 parts by weight of a polymerization initiator relative to 100 parts by weight of the vinyl aromatic monomer are dispersed in an aqueous medium. Then, the resulting suspension is heated to such a temperature that the monomer does not substantially polymerize to impregnate the inside and the surface of the polyethylene-based resin particles with the monomer. Subsequently, the temperature of the suspension is raised to polymerize the monomer and a vinyl aromatic polymer is micro-dispersed in the polyethylene-based resin to obtain the expanded molded article of modified polyethylene-based resin.
In the above patent, there is no disclosure on linear low-density polyethylene obtained by using a metallocene compound as a catalyst. The non-crosslinked linear low-density polyethylene-based resin particles (ULT-ZEX 3021F manufactured by Mitsui Chemicals, Inc.) described in its Examples are linear low-density polyethylene obtained by polymerization using an ordinary Ziegler-Natta catalyst.
[Patent Document 1] Japanese Examined Patent Publication No. SHO 51(1976)-46138
[Patent Document 2] Japanese Examined Patent Publication No. SHO 52(1977)-10150
[Patent Document 3] Japanese Examined Patent Publication No. SHO 58(1983)-53003
[Patent Document 4] Japanese Unexamined Patent Publication No. SHO 6 (1987)-59642
[Patent Document 5] Japanese Patent No. 2668384